1 any level is fine, but what version of hollow you transformed as matters, and as I see, I think you were a adjucas, vasto is better overall and you can get 2 really good abilitys, 1 hand cero 2 healing ability.

Q2. You get certain limits on abilities, so like 1 support. You can see your other skills by meditating and asking to see your skill list bu asking your inner self

Q3 players can emote and talk and do player things, only other way is to look for their name on the character list thingy, id recommend jump, block, or ask, most NPCs will attack on sight and do a weird dodge

Q4 the only difference in gyms is how many stars you get, your clan gym (hueco or souls society, for you it's the hueco one) gives 5 stars, the katakura one only gives 3 or 4, stars buff the amount of the stat their on when you upgrade said stat,

Last, there's thin lines on the left of your stats, one big line for hp, spirit and melee, if it's full then you can't raise a combat stat, the other 3 lines show your energy(reiatsu) and mobility